The present invention relates generally to solid/fluid separation systems and is more particularly directed to a new and improved filter system of the pneumatic hydro-pulse type employing repeatably recleanable filter elements and to a new and improved method of operating such a system.
In prior pneumatic hydro-pulse filter systems, the feed slurry of a separable solid/fluid mix flows into the bottom of a tank holding an array of vertical tubular filter elements that are closed at their bottom ends. The slurry comes into contact with the exterior surface of the tubes causing the solid material to be retained on the exterior of the filter elements while the liquid or fluid passes therethrough into the interior of the tubes and subsequently out the top of the tank. The filtered solids restrained by the tubes form a loose cake on the exterior of the filter elements. These outside-in filter systems typically use a low fluid approach velocity so as not only to provide a surface filtration effect with restricted filter intrusion but also to permit a pulsed backflow radially through the filter elements to remove the filtered solids on the exterior of the filter elements. A sudden pulse that forces filtrate through the walls of the filter elements is effective to "blow" or dislodge the cake from the exterior of the filter surfaces. The dislodged solids are then drained from the tank structure or housing, by totally evacuating all the fluid within the housing. Thereafter the filtration process is restarted.
These prior outside-in systems exhibit several drawbacks such as the uneven or incomplete discharge of solids on the exterior of the filter tubes resulting in an undesirable deterioration of fluid velocity along the length of the filter tubes. Additionally, the prior systems evacuated the entire housing during flushing thereby using and losing an excessive amount of liquid during the flushing operation. They also exhibited a complete lack of versatility for various filtration applications and required a shut down in the feed line. However the most notable deficiencies come to light with respect to the filtering of dispersed fibrous solids. The fibers tended to form a wet mat about the exterior of the filter elements. This mat does not satisfactorily separate or blow free from the filter elements upon application of the hydraulic pulse to permit the requisite draining of the accumulated fibrous mats. The fibrous material that does separate may reslurry or remain in clumps and tends to "hang up" on the frame structure supporting the filter elements, clogging the upstream side of the filter unit.
In accordance with the present invention it has been found that these and related disadvantages can be obviated by providing a new and improved filter system and method of operation that utilizes an axial flushing action through the center of open ended filter tubes and provides substantially greater versatility and flexibility of operation. In such a system the solid/fluid slurry first engages the inside, rather than outside, walls of the porous filter tubes. The inside walls provide surfaces that are free of structural support members and are open ended so as to communicate directly with both the inlet and solid discharge areas of the system. Since the particles are not collected on the outside of the filter elements, more elements can be used within the same space thereby achieving greater filtering surface area and a more efficient axial discharge operation. A pressurized backflow pulse of fluid that moves radially through the tubes tends to lift the layer of accumulated solids from the surface of the tubes as a cohesive mass while providing a thin fluid film between that mass and the interior surface of the tubular filter elements. The film acts as a fluid bearing that cooperates with the axial discharge flow to allow easy and efficient discharge while advantageously using less filtrate for flushing the solids from the system.
Another feature of the invention is its ability to accommodate a wide variety of operating modes utilizing flow from both the top and bottom of the system as well as recirculating flow, all with improved efficiency, and its adaptability to a wide variety of filtering applications with the collected solids discharged as a slurry, a semi-dry paste or a fully dry material. Included is the provision for a filter system that is efficient and economical in operation, variable in its ability to accommodate multiple units of the same or different operating modes yet can be adjusted to preserve and reclaim either the solid, the filtrate or both with increased filtering efficiency.
Other features and advantages will be in part obvious and in part pointed out more in detail hereinafter.
It has now been found that the foregoing and related objects and advantages can be readily obtained in a filter system and method of the type described that includes the provision for an elongated filter array comprising a plurality of tubular filter elements having interior wall surfaces forming interior open ended axial bores, the introduction of a flow stream of a solid/fluid feed slurry into simultaneous operative engagement with the interior wall surfaces of each of said tubular filter elements of the array at a sufficient pressure and controlled velocity so that the fluid of the slurry passes outwardly through the walls of the tubular filter elements and the solids within the slurry are collected as a cake on the wall surfaces without substantial intrusion into the porous filter element, and the provision for a discharge fluid flow axially through the interior of the tubular elements along the full length of the cake for removal of the cake of solids axially along the interior of the tubular filter elements and out the open end thereof. The system can employ a sudden pressure pulse that is applied to the cake to dislodge the filtered solids from the interior tube surface, and assist in the rapid axial discharge of the dislodged solids out of the open bores of the filter tube elements. The pressure pulse can provide a periodic radial backflow pulse of fluid inwardly through the cylindrical walls of the filter tubes to dislodge the filter solids from the interior surface of the cylindrical walls while a discharge outlet assists in the pulsing operation. The versatility of the system lends itself to numerous and varied modes of operation, including the use of multiple associated units operated in similar or different modes.
A better understanding of the invention will be obtained from the following detailed description and accompanying drawings which set forth the several steps and the relation of one or more of such steps with respect to each of the others and the system possessing the features, characteristics, properties, and the relation of elements exemplified herein.